1. Field of the Invention
This invention relates to electrophotographic document copiers and/or printers and more particularly to automatic adjustment of parameters influencing reproduction of such copiers and/or printers.
2. Description of the Prior Art
In electrophotographic copiers and/or printers, contrast density and color balance (in color machines) can be adjusted by changing certain process control parameters such as primary voltage V.sub.o, exposure E.sub.o and development station bias voltage V.sub.B, the concentration of toner in the development mixture and the image transfer potential.
Control of such parameters is often based on measurements of the density of a toner image in a test patch. Typically, the test patch can be recorded on an area of the electrophotoconductive imaging member between adjacent image frames and developed. The developed density of the patch can be measured and adjustments made accordingly.
In U.S. Pat. No. 5,087,942, there is disclosed a copier for copying transparencies wherein a patch is optically exposed on a photoconductor using a first light source and developed. The density of the patch is measured and compared to target values set during manufacture to maintain process control parameters. In order to adjust E.sub.o, the patent suggests that E.sub.o be adjusted by comparing the measured density value with aim density values and adjusting the illumination from a second light source that is used to illuminate the transparency for making the reproduction. A problem with this approach is the use of a separate light source to record the patches since it is desirable to have closed loop control of the exposure by having the same exposure source that is creating the patch be used for printing the images. Another drawback is that only the E.sub.o control parameter is adjusted. While this approach may regulate a single density level well, good regulation of the entire tone scale generally requires adjustment of at least 2 process control parameters; e.g., E.sub.o and V.sub.o, and V.sub.B.
U.S. Pat. No. 4,647,184 discloses an electrophotographic printing apparatus which includes controls for establishing basis xerographic parameters to produce optimum copy quality. In this apparatus, adjustments in exposure and charging parameters are provided for by producing patches at a maximum density, an intermediate density and a minimum density. An undesirable feature of this type of control is the added complexity of rendering multiple patches at different densities, measuring the different densities, providing calculations at each of the measured densities and then providing an iterative process for which optimum values for the parameters are obtained.
U.S. Pat. No. 5,436,705 discloses an adaptive controller for controlling a plurality of process parameters in an electrophotographic printing machine. A toner area coverage sensor detects a plurality of different density level values for a toner image and generates corresponding signals. These signals are compared with reference signals at each of the density levels and the differences or errors are input to a linear quadratic controller to compute new values to provide adjustments to the various parameters. Again, the use of patches at various density levels provides an added complexity which requires wasting of toner and provides extra wear on the cleaning apparatus which is operated to remove toned patches from the photoconductor. Furthermore, customer image productivity may be compromised in order to print the multiple test patches.
U.S. Pat. No. 4,853,738 also discloses the use of multiple test patches of different densities for controlling two adjustable process control parameters such as V.sub.o and E.sub.o. A complex calculation involving a matrix of values associated with each of the measured densities adds calculation complexity and, as noted above, requires waste of toner and extra wear on the cleaning apparatus.
It is therefore an object of the invention to provide a process control method and apparatus which compensates well for tone scale shifts caused by changing environmental conditions and rest/run effects, requires fewer printed process control patches than other feedback strategies, reduces range requirements for V.sub.o and E.sub.o. and is robust over material variations, i.e., different toner compositions.